1. Field of the Invention
The present invention relates to a gamma-ray compensated ionization chamber used for counting only neutrons under the presence of gamma rays in a nuclear reactor or the like.
2. Description of the Related Art
FIG. 1 is a schematic diagram showing the construction of a conventional gamma-ray compensated ionization chamber (hereinafter abbreviated as CIC) depicted, for example, in "Measurements of Nuclear Radiation" (pp. 98-102, Nikkan Kogyo Shimbun, Inc., 1978). As shown in the FIGURE, the CIC 10 comprises three cylindrical electrodes disposed concentrically: a high-voltage electrode 1 as the outermost electrode; a signal electrode 21 disposed inwardly spaced apart from the high-voltage electrode 1; and a compensating electrode 31 disposed inwardly spaced apart from the signal electrode 21. A high-voltage power supply 4 applies a high voltage +V.sub.H to the high-voltage electrode 1, while a compensating power supply 5 applies a compensating voltage -Vc to the compensating electrode 31. The neutron current obtained from the signal electrode 21 is amplified through an amplifier 6.
The inside surface of the high-voltage electrode 1 and the outside surface of the signal electrode 21 are coated with a neutron-sensitive material 7 such as .sup.10 B. The outside surface of the compensating electrode 31 is provided with a plurality of circumferentially extending grooves 8.
In operation, when the gamma rays emitted by the nuclear reaction and neutrons are radiated to the CIC 10, a neutron current In caused by the neutron-sensitive material 7 and a current I.gamma..sub.1 caused by the gamma rays flow through the space (neutron ionization chamber) between the high-voltage electrode 1 and the signal electrode 21. At the same time, another gamma-ray current I.gamma..sub.2 flows through the space (compensating ionization chamber) between the signal electrode 21 and the compensating electrode 31.
Because the applied currents from the high-voltage power supply 4 and the compensating power supply 5, respectively connected to the high-voltage electrode 1 and the compensating electrode 31, are opposite in polarities to each other, the ionization currents flowing in the two respective chambers flow in opposite directions to each other with respect to the signal electrode 21. When the current flowing to the signal electrode 21 is denoted as Isig, the following equation is given. EQU Isig=In+I.gamma..sub.1 -I.gamma..sub.2 ( 1)
If I.gamma..sub.1 and I.gamma..sub.2 are equal, we have EQU Isig=In (2)
Thus, the net neutron current without containing the gamma-ray current can be measured. This operation is the gamma-ray compensation employed in the CIC 10. The neutron current In is detected from the signal electrode 21 and is amplified through the amplifier 6.
Since an ionization chamber in which I.gamma..sub.1 and I.gamma..sub.2 are exactly equal is difficult to construct, a large number of narrow grooves 8 are provided on the outer circumferential surface of the compensating electrode 31 so as to exactly equalize I.gamma..sub.2 to I.gamma..sub.1 by controlling the voltage -Vc supplied from the compensating power supply 5. The reason that I.gamma..sub.2 is varied by the grooves 8 on the outside surface of the compensating electrode 31 is as follows.
Since the electric field strength inside the groove 8 is weak, ionized electrons are not completely collected so that the portion of the electrode 31 having the grooves 8 is ineffective an does not effectively function as an ionization chamber. The volume of this ineffective portion depends on the compensating voltage Vc supplied from the compensating power supply 5; it decreases as the voltage Vc increases. Therefore, the effective volume of the ionization chamber is varied by controlling the compensating voltage Vc so as to equalize I.gamma..sub.2 to I.gamma..sub.1.
In the above construction of the conventional gamma-ray compensated ionization chamber, the gamma ray current is compensated by controlling the compensating voltage Vc. The volume of the ineffective portion resulting from the weak field strength inside the grooves 8 is unstable, however, because the compensating conditions vary with changes in external conditions such as the intensity and energy of gamma rays, thus resulting in overcompensation or undercompensation.